High frequency light source brings optical computing closer

A new ultrafast light-emitting device that can switch on and off 90 billion times a second could form the basis of future optical computing.

Engineers from Duke University in the USA have created a so-called ‘plasmonic device’ using semiconductor quantum dots that can emit light at more than 90 billion gigahertz.

Being able to switch a light source on and off rapidly is the first step towards creating electronics that use photons instead of electrons to process and transmit data, which would allow computers to operate much faster.

Lasers are able to reach very high frequencies but they are too energy-hungry and unwieldy to integrate into computer chips. The device created at Duke, however, is only nanometres across making it far more suited to integration into electronic circuits.

"This is something that the scientific community has wanted to do for a long time," said Maiken Mikkelsen, an assistant professor of electrical and computer engineering and physics at Duke.

"We can now start to think about making fast-switching devices based on this research, so there's a lot of excitement about this demonstration."

In a study published today in Nature Communications the researchers described how they shone a laser at the surface of a silver cube just 75 nanometres wide, causing the free electrons on its surface begin to oscillate together in a wave.

These oscillations create their own light, which reacts again with the free electrons trapping energy on the surface of the nanocube. This trapped energy creates quasi-particles called plasmons, which cause an intense electromagnetic field to develop between the silver nanocube and a thin sheet of gold placed a mere 20 atoms away by the researchers.

Quantum dots – spheres of semiconducting material just six nanometres wide – positioned between the nanocube and the gold interact with the field to produce a directional, efficient emission of photons that can be turned on and off at a frequency of more than 90 gigahertz.

"There is great interest in replacing lasers with LEDs for short-distance optical communication, but these ideas have always been limited by the slow emission rate of fluorescent materials, lack of efficiency and inability to direct the photons," said Gleb Akselrod, a postdoctoral researcher in Mikkelsen's laboratory. "Now we have made an important step towards solving these problems."

The group is now working on using the plasmonic structure to create a single photon source – a necessity for extremely secure quantum communications – by sandwiching a single quantum dot in the gap between the silver nanocube and gold foil.

They are also trying to precisely place and orient the quantum dots to create the fastest fluorescence rates possible.

"The eventual goal is to integrate our technology into a device that can be excited either optically or electrically," said Thang Hoang, also a postdoctoral researcher in Mikkelsen's laboratory. "That's something that I think everyone, including funding agencies, is pushing pretty hard for."